This invention relates to knitting machines, and more particularly to circular knitting machines of the double cylinder type suitable for the production of stockings, socks and like tubular hosiery or garments. More particularly, the invention deals with provisions in such a double cylinder circular knitting machine for drawing and holding under tension the fabric being fashioned.
The double cylinder circular hosiery knitting machine is per se well known in the art. It comprises an upper and a lower rotatable needle cylinder each carrying a set of needles and disposed upstandingly, in end-to-end relation with each other, to form stitches therebetween. The fabric being knitted is drawn by suction into the upper needle cylinder.
Usually, the knitting machine of this general character includes a device for drawing up and holding under tension the knitwork being formed, in order to avoid stitch irregularities, among other purposes. Japanese Unexamined Patent Publication No. 63-75156 describes and claims an example of such knitwork drawing device.
This prior art device comprises a fabric locking assembly having a set of retractable locking fingers and itself disposed within the lower needle cylinder for movement in its axial direction into and out of the upper needle cylinder. For such movement of the fabric locking assembly an air cylinder is provided which is mounted vertically within the lower needle cylinder and which has an elongate, hollow piston rod coupled at one end to the fabric locking assembly. The other end portion of the hollow piston rod is slidably fitted over a fixed air nozzle. The air cylinder when extended moves the fabric locking assembly into close engagement, via the fabric being knitted, with the bottom end of an inner movable tube concentrically nested within the upper needle cylinder. The upper needle cylinder also accommodates an outer movable tube which is slidably fitted over the inner movable tube for relative axial displacement.
Following the fabric engagement by the fabric locking assembly against the bottom end of the inner movable tube, air under pressure is supplied through the air nozzle into the hollow piston rod and thence into the fabric locking assembly. So actuated pneumatically, the fabric locking assembly has its locking fingers spread apart for additionally locking the fabric against the inside surface of the inner movable tube.
Both air cylinder and fabric locking mechansim are maintained pressurized during the progress of knitting. Consequently, the fabric locking assembly travels up with the inner movable tube into the upper needle cylinder under air pressure, thereby stretching the fabric being formed. The outer movable tube within the upper needle cylinder, on the other hand, is pneumatically lowered in step with the ascent of the inner movable tube for depressing the fabric.
A primary objection to this known knitting machine is the complexity of its construction and operation. During knitting, not only must the upper and lower needle cylinders and the inner and outer movable tubes be synchronously rotated about a common axis, but also the inner and outer movable tubes must be axially moved in the opposite directions. The fabric tensioning device must also operate in step with such complex movements of the various machine components for imparting tension to the fabric being knitted.
Another objection is that the fabric locking assembly substantially airtightly closes the bottom end of the inner movable tube upon locking the fabric against it. Consequently, suction becomes no longer exerted on the fabric received in the inner movable tube.
An additional disadvantage of the prior art manifests itself when the article to be produced is longer than the allowed stroke of the fabric locking assembly. The locking assembly must then temporarily release the fabric and must be lowered to the initial position for reengaging a lower part of the fabric. The prior art device has been incapable of holding the fabric stretched during such fabric reengagement, so that the operation of the knitting machine has had to be suspended. Such suspensions of knitting operation not only decreases the production of the machine but can also gives rise to uneven stitches through a difference in tensile force exerted before and after each suspension.